1. Field of the Invention
The present invention relates to a piezoelectric film loudspeaker using a flexible piezoelectric film with electrodes formed on both surfaces thereof as a diaphragm.
The present application claims priority from Japanese Patent Application No. 2001-336201, the disclosure of which is incorporated herein by reference for all purposes.
2. Description of the Related Prior Art
In recent years, according to downsizing and thinning a stereophonic apparatus, a television apparatus or the like, a loudspeaker as a sound outputting equipment for them also has been requested to be downsized, thinned down, and lightened. In particular, with regard to the loudspeaker for automobiles, a more compact loudspeaker is needed since there has been requested an enough livability within a vehicle. In addition, since it has been requested for all passengers on the seats to uniformly enjoy sound of an even strength, it is proposed to mount a piezoelectric film loudspeaker which has a thin shape and is light in weight within a vehicle, for example, using a wide and flat surface such as a ceiling of the vehicle.
The conventional piezoelectric film loudspeaker has a shape as shown in FIGS. 6a, 6b, 7a and 7b. FIGS. 6a and 7a are side views, and FIGS. 6b and 7b are plan views. The piezoelectric film loudspeaker 60 as shown in FIGS. 6a and 6b has a diaphragm of a a high polymer piezoelectric film 62 with electrodes formed on both surfaces thereof, a curved state of the high polymer piezoelectric film 62 of which is supported by side frames 61 made of metal or plastic. Signals from a terminal 63 are applied to the electrodes formed on the surfaces of the high polymer piezoelectric film 62, thereby generating sound from the surface of the high polymer piezoelectric film 62.
The piezoelectric film loudspeaker 70 as shown in FIGS. 7a and 7b is provided with a high polymer piezoelectric film 72 with electrodes formed on both surfaces thereof, which abuts on four convex and elastic support bodies. The connecting portion of each elastic support body is provided with a support member 74. The other structures such as a frame 71 and a terminal 34 are the same as ones of FIGS. 6a and 6b. The piezoelectric film loudspeaker 70, therefore, can be bent at each portion of support members 74, thereby enabling the piezoelectric film loudspeaker 70 to be attached while being bent and transformed in accordance with the shape of the ceiling or the like in the vehicle.
In the conventional piezoelectric film loudspeaker as mentioned above, the electrodes are formed on the both whole surfaces, i.e., front and rear surfaces of the single high polymer piezoelectric film 62, 72. That is, the diaphragm, in which the whole surface of the high polymer piezoelectric film 62, 72 is vibrated by signals from the terminal 63, 73, is formed. Therefore, equivalent circuits for the piezoelectric film loudspeaker are as shown in FIGS. 8a and 8b. FIG. 8a shows the equivalent circuit without a transformer, and FIG. 8b shows one with the transformer. In these cases, the signals are supplied from a signal source S to a capacity C which is formed by the single high polymer piezoelectric film 62, 72 directly or through the transformer T.
According to such a conventional piezoelectric film loudspeaker, a plane wave with a single phase can be generated for a desired space since a single surface of diaphragm is utilized, thereby allowing the even level of sound pressure to be gained over the desired space. However, it is difficult for the diaphragm of the single surface of high polymer piezoelectric film to output all the bands (20-20000 Hz) like a dynamic loudspeaker, even if the even level of sound pressure can be output. In particular, the conventional piezoelectric film loudspeaker, in which the periphery of the loudspeaker is supported by the frame and thus the vibration of flat surface is caused, has problems that the distortion of frequency characteristic may occur since a divided vibration is apt to occur when the reproducing frequency of the loudspeaker with the relatively large area is set to be high. Additionally, in the diaphragm which unifiedly supplies signals to the single surface, there is also a problem that the degree of freedom for designing each kind of characteristic is little.
In order to settle these problems, one way is to combine a plurality of high polymer piezoelectric film loudspeakers, wherein each area of the high polymer piezoelectric film is kept to be small. However, in this case, it is difficult to arrange the phase of each piezoelectric film loudspeaker when setting it, and also the increase of cost is caused.